In controlling the current flow to high power devices, transistors have been used typically in one of two modes. One approach is to use a transistor in a switching mode and employ a large inductor to smooth the flow of current. If the frequency of the switching circuit is fixed, then the average current may be controlled by adjusting the period of time the transistor is conducting. In this mode, the power rating of the transistor is usually limited by its breakdown voltage and switching time. In another mode, transistors may be operating in a linear mode with a plurality of transistors connected in parallel and provided with cooling means. The power rating of the transistor is a function of voltage and current, however the power rating may be reduced as the voltage is increased due to a characteristic known as secondary breakdown. If the critical voltage at any given current level is exceeded, then the transistor is likely to fail.
In order to handle large currents from high voltage sources, a plurality of transistor arrays may be connected in series with the hope that the voltage will be distributed in such a way that each transistor will be operated within its limitations. However, such arrangements are difficult to balance, and the failure of one or more transistors within one array may result in a higher voltage being placed across the other arrays in series therewith, causing those transistors to fail because of the above mentioned secondary breakdown characteristics. The most common failure mode for transistors operated in this service is to become short circuited.
In the event of a short circuit of a transistor within one array, the over voltage condition experienced by the other transistors may be so severe that the other transistors will be damaged before a circuit breaker or other conventional type of protection circuit could operate. Ultra sophisticated and complicated protection circuits are available and might be useful, but they would also be expensive.
In recent years, development of electric vehicles has resulted in the employment of battery arrays of higher and higher voltages in order to reduce the current and thereby reduce losses in the wiring and control functions. These batteries ought to be tested under conditions which simulate actual use. By way of example, one battery array may include as many as eighteen batteries which have a nominal six-volt rating. It is understood, however, that a fully charged cell may have a full charge potential of 2.75 volts, and therefore each battery will have a 8.25 volt output, and an entire array, when fully charged, will be 148.50 volts. This voltage is far in excess of the voltage rating of most transistors presently available for operation in the linear mode for charging or discharging batteries at a controlled rate.